Transducers using piezoelectric technologies are used for precise positioning at the nanometer scale. Typically, piezoelectric devices include a ceramic that is formed into a capacitor that changes shape when charged and discharged. These piezoelectric devices can be used as position actuators because of their shape changing properties (i.e., vibrations). When such a piezoelectric device is used as a position actuator, the shape change of the ceramic is approximately proportional to an applied voltage differential across the ceramic.
Several types of resonant motor systems and resonant actuator systems use piezoelectric generated vibrations to create continuous movement of elements with high speed, high torque, small size, and quiet operation. An exemplary prior art motor is a linear motor system that includes a threaded element or nut. The threaded element includes four symmetrically positioned piezoelectric transducers or members. Driving signals drive the transducers to simultaneous excite the orthogonal bending modes of the threaded element at a first bending mode resonant frequency. The driving signals are typically in the ultrasonic range with a plus or minus ninety-degree phase shift to generate a circular orbit. The threaded element orbits a threaded shaft at the first bending mode resonant frequency, which generates torque that rotates the threaded shaft that moves the threaded shaft linearly.
Examples of the above resonant motor systems and resonant actuator systems may be found in U.S. Pat. No. 6,940,209, entitled, “Ultrasonic Lead Screw Motor”; U.S. Pat. No. 7,339,306, entitled, “Mechanism Comprised of Ultrasonic Lead Screw Motor”; U.S. Pat. No. 7,170,214, entitled, “Mechanism Comprised of Ultrasonic Lead Screw Motor”; and U.S. Pat. No. 7,309,943, entitled, “Mechanism Comprised of Ultrasonic Lead Screw Motor,” all of which are commonly assigned to New Scale Technologies, Inc. and are all hereby incorporated herein by reference in their entireties.
A controller typically generates and supplies one or more driving signals to drive the piezoelectric transducers at a fixed driving frequency. The fixed driving frequency is typically selected to be close to a known or estimated nominal mechanical resonant frequency of the actuator system. Driving the piezoelectric transducers at such a nominal resonant frequency can increase the actuator's overall performance and efficiency. Increases in performance can include faster rotational and linear speeds and larger push forces. However, the resonant frequency of these actuators change based on variables including, but not limited to, ambient temperature, motor temperature, loading and manufacturing tolerances. Thus, driving a motor system and/or an actuator system with a fixed driving frequency can result in diminished performance over time. This loss in performance can cause the actuator to be less efficient, waste energy, run at slower than desired or optimal speeds, fail to move a specific load, and add strain to the motor system and/or actuator system.
Heretofore, some patents and publications have disclosed methods for driving resonant actuator devices, which may be briefly summarized as follows:
U.S. Pat. No. 5,233,274 to Honda et al. discloses a drive circuit used in a Langevin type ultrasonic bolt-tightening motor in which a motor drive voltage having a given frequency is applied to a piezo-electric element in a stator section, the resulting longitudinal and torsional vibrations being effective to rotate a motor section. The drive circuit has a longitudinal vibration sensor for detecting the longitudinal vibration in the stator section, a torsional vibration sensor for detecting vibration in the stator section and a frequency controller for controlling the frequency of the motor drive voltage such that the phase difference between the detection signals of the longitudinal and torsional vibration sensors becomes 90 degrees. The frequency of the motor drive voltage can be feedback controlled to maintain an optimum drive frequency despite the varying of the optimum drive frequency due to changes in various factors. The disclosure of this patent is incorporated herein by reference.
United States Patent Application Publication No. 2008/0129145 to Lee et al. discloses a piezoelectric actuator for driving a piezoelectric unit having two resonance points. The piezoelectric unit includes an optimal driving frequency calculating unit that adds a delta frequency, having a constant frequency difference from a first resonant frequency of the piezoelectric unit, to a characteristic resonant frequency obtained by analyzing characteristics of the piezoelectric unit, thereby calculating an optimal driving frequency; and an FM modulating unit that is connected to the optimal driving frequency calculating unit and generates the optimal driving frequency, calculated by the optimal driving frequency calculating unit, so as to supply to the piezoelectric unit. The disclosure of this published patent application is incorporated herein by reference.
United States Patent Application Publication No. 2009/0009109 to Hashimoto discloses a method for driving an ultrasonic motor having an actuator section. The method includes a step of starting the ultrasonic motor by applying an AC voltage with a first frequency to the actuator section; a voltage detection step of detecting a voltage generated at the actuator section while lowering a driving frequency from the first frequency to a second frequency at which the ultrasonic motor stops; a starting step of starting the ultrasonic motor with a third frequency; and a driving step of changing the driving frequency from the third frequency to a lower frequency such that the driving frequency has a value within an operation frequency range. The disclosure of this published patent application is incorporated herein by reference.